Methods and Apparatus for Low Voltage Control of a Subscriber Line Interface Circuit

1. An apparatus comprising: a low voltage subscriber line interface circuit (SLIC) to control a plurality of SLIC functions at a low voltage, the low voltage SLIC including: a voltage detection circuit having a first pair of operational amplifiers each to receive one of a tip and ring voltage of a subscriber loop and output a scaled version; and a second pair of operational amplifiers each coupled to receive the scaled versions of the tip and ring voltages and to generate an overvoltage signal if the tip voltage or ring voltage is greater than a predetermined threshold.

2. The apparatus of claim 1 , wherein the voltage detection circuit further comprises a switchable shunt resistance coupled between the inputs and outputs of each of the first pair of operational amplifiers, the switchable shunt resistance controllably switched into the voltage detection circuit if the tip voltage or ring voltage is greater than a trigger level.

3. The apparatus of claim 1 , wherein the low voltage SLIC further comprises a longitudinal voltage controller including a first operational amplifier to receive the scaled versions of the tip and ring voltages at a first input and a scaled battery voltage at a second input, the first operational amplifier to generate a longitudinal voltage for use by a high voltage SLIC coupled to the low voltage SLIC.

4. The apparatus of claim 3 , wherein the low voltage SLIC further comprises a third pair of operational amplifiers coupled to the output of the first operational amplifier, wherein the third pair of operational amplifiers is to generate a fault signal based on comparison of the longitudinal voltage to a first threshold and a second threshold.

5. The apparatus of claim 1 , wherein the low voltage SLIC further comprises a bias generator to generate a plurality of low voltages, the bias generator including a first low voltage operational amplifier having an output coupled to a first resistor string to generate a plurality of positive reference voltages and a second low voltage operational amplifier coupled to the output of the first low voltage operational amplifier and having an output coupled to a second resistor string to generate a plurality of negative reference voltages.

6. The apparatus of claim 5 , further comprising an external resistor coupled to a first input of the first low voltage operational amplifier and a reference voltage coupled to a second input of the first low voltage operational amplifier, wherein the external resistor is to track on-chip sheet resistance of the first resistor string.

7. The apparatus of claim 1 , wherein the low voltage SLIC further comprises a DC feed circuit to provide a metallic control current to a high voltage SLIC coupled to the low voltage SLIC, wherein the DC feed circuit comprises: a first operational amplifier to receive the scaled versions of the tip and ring voltages to compare to a voltage representative of a DC open circuit voltage or a ringing voltage of the subscriber loop; and a second operational amplifier having a first input coupled to an output of the first operational amplifier to amplify the output of the first operational amplifier to generate the metallic control current.

8. The apparatus of claim 7 , further comprising a current limiter coupled between the output of the first operational amplifier and the first input of the second operational amplifier to limit the metallic control current, wherein the current limiter comprises a fourth pair of operational amplifiers each to receive the output of the first operational amplifier and a programmable reference voltage.

9. The apparatus of claim 8 , wherein the current limiter is to limit the first input of the second operational amplifier of the DC feed circuit to a value of a selected one of a plurality of programmable reference voltages.

10. The apparatus of claim 7 , further comprising a plurality of switchable resistances to be switched into the DC feed circuit during a ringing mode of the high voltage SLIC.

11. An apparatus comprising: a low voltage subscriber line interface circuit (SLIC) to perform a plurality of SLIC functions at a low voltage and to provide a metallic current and a longitudinal current to a high voltage SLIC, wherein the low voltage SLIC includes a plurality of circuits to perform the SLIC functions, wherein each of the plurality of circuits is based on an operational amplifier cell architecture; and the high voltage SLIC coupled to the low voltage SLIC to receive the metallic current and the longitudinal current and to generate a first amplified output current and a second amplified output current from the metallic current and the longitudinal current, the high voltage SLIC comprising: a first bidirectional current amplifier formed of a first pair of unidirectional current amplifiers; and a second bidirectional current amplifier formed of a second pair of unidirectional current amplifiers.

12. The apparatus of claim 11 , wherein a first one of the plurality of circuits comprises a longitudinal voltage controller including a first operational amplifier to receive scaled versions of tip and ring voltages of a subscriber loop at a first input and a scaled battery voltage at a second input, the first operational amplifier to generate a longitudinal voltage for use by the high voltage SLIC.

13. The apparatus of claim 12 , wherein a second one of the plurality of circuits comprises a voltage detection circuit having a first pair of operational amplifiers each to receive one of the tip and ring voltage and output the scaled version and a second pair of operational amplifiers coupled to receive the scaled versions of the tip and ring voltages and to generate an overvoltage signal if the tip voltage or ring voltage is greater than a predetermined threshold.

14. The apparatus of claim 13 , wherein the second one of the plurality of circuits further comprises a longitudinal voltage controller including a third operational amplifier to receive the scaled versions of the tip and ring voltages at a first input and a scaled battery voltage at a second input, the third operational amplifier to generate the longitudinal voltage.

15. The apparatus of claim 13 , wherein a third one of the plurality of circuits comprises a DC feed circuit to provide a metallic control current to the high voltage SLIC, the DC feed circuit including a fourth operational amplifier to receive the scaled versions of the tip and ring voltages to compare to a voltage representative of a DC open circuit voltage or a ringing voltage of the subscriber loop, and a fifth operational amplifier having a first input coupled to an output of the fourth operational amplifier to amplify the output of the fourth operational amplifier to generate the metallic control current.

16. The apparatus of claim 11 , wherein the low voltage SLIC and the high voltage SLIC are formed on a single substrate of an integrated circuit.

17. The apparatus of claim 11 , further comprising an analog interface to couple the low voltage SLIC and the high voltage SLIC, wherein the analog interface comprises a three-wire interface.

18. The apparatus of claim 17 , wherein the high voltage SLIC is to interface with system chips of different process technologies via the analog interface.

19. A system comprising: a digital signal processor (DSP) to perform signal processing on voiceband communication signals, the DSP coupled between a central office and a subscriber loop, the DSP including a circuit block to perform a plurality of low voltage subscriber line interface circuit (SLIC) functions, each of the low voltage SLIC functions performed by circuits having an operational amplifier-based cell architecture, wherein a first circuit of the circuit block comprises a switching regulator feedback controller having a first pair of low voltage operational amplifiers each to receive scaled versions of tip and ring voltages of the subscriber loop, the first pair of low voltage operational amplifiers having outputs coupled to a first input of a third low voltage operational amplifier, the third low voltage operational amplifier to output a control signal for the switching regulator; and a high voltage SLIC coupled to the DSP via an analog interface, the high voltage SLIC including a first current amplifier and a second current amplifier to amplify currents received from the circuit block via the analog interface, wherein an output of the high voltage SLIC is coupled to the subscriber loop.

20. The system of claim 19 , further comprising a current source to provide an offset current to the first input of the third low voltage operational amplifier, the first input further coupled to receive a scaled battery voltage.

21. The system of claim 19 , wherein a second circuit of the circuit block comprises a ring detector to detect off hook and ringing modes on the subscriber loop, the ring detector including: a second pair of low voltage operational amplifiers each to receive one of a set of programmable reference voltages and an input signal, and to output a bidirectional signal in the ringing mode; and a fourth low voltage operational amplifier having a first input to receive a filtered version of the bidirectional signal and to output a detection signal.